Fluorocarbon based fluids have found widespread use in industry for refrigeration, air conditioning and heat pump applications.
Vapor compression is one form of refrigeration. In its simplest form, vapor compression involves changing the refrigerant from the liquid to the vapor phase through heat absorption at a low pressure and then from the vapor to the liquid phase through heat removal at an elevated pressure. First, the refrigerant is vaporized in the evaporator which is in contact with the body to be cooled. The pressure in the evaporator is such that the boiling point of the refrigerant is below the temperature of the body to be cooled. Thus, heat flows from the body to the refrigerant and causes the refrigerant to vaporize. The vapor formed is then removed by means of a compressor in order to maintain the low pressure in the evaporator. The temperature and pressure of the vapor are then raised through the addition of mechanical energy by the compressor. The high pressure vapor then passes to the condenser whereupon heat exchanges with a cooler medium. The sensible and latent heats are removed with subsequent condensation. The hot liquid refrigerant then passes to the expansion valve and is ready to cycle again.
While the primary purpose of refrigeration is to remove energy at low temperature, the primary purpose of a heat pump is to add energy at higher temperature. Heat pumps are considered reverse cycle systems because for heating, the operation of the condenser is interchanged with that of the refrigeration evaporator.
Certain chlorofluorocarbons have gained widespread use in refrigeration applications including air conditioning and heat pump applications owing to their unique combination of chemical and physical properties. The majority of refrigerants utilized in vapor compression systems are either single component fluids or azeotropic mixtures. Single component fluids and azeotropic mixtures are characterized as constant-boiling because they exhibit isothermal and isobaric evaporation and condensation. The use of azeotropic mixtures as refrigerants is known in the art. See, for example, R. C. Downing, "Fluorocarbon Refrigerants Handbook", pp. 139-158, Prentice-Hall, 1988, and U.S. Pat. Nos. 2,101,993 and 2,641,579.
Azeotropic or azeotrope-like compositions are desired because they do not fractionate upon boiling or evaporation. This behavior is desirable because in the previously described vapor compression equipment with which these refrigerants are employed, condensed material is generated in preparation for cooling or for heating purposes, and unless the refrigerant composition is constant boiling, i.e. is azeotrope-like, fractionation and segregation will occur upon evaporation and condensation and undesirable refrigerant distribution may act to upset the cooling or heating.
Non-azeotropic mixtures have been disclosed as refrigerants, see, e.g., U.S. Pat. No. 4,303,536, but have not found widespread use in commercial applications even though the ability of non-azeotropic refrigerant blends to exhibit improved thermodynamic performance has often been discussed in the literature. See, e.g., T. Atwood, "NARBS--The Promise and the Problem", American Society of Mechanical Engineers, Winter Annual Meeting, paper 86-WA/HT-61, 1986 and M. O. McLinden et al., "Methods for Comparing the Performance of Pure and Mixed Refrigerants in the Vapor Compression Cycle", Int. J. Refrio. 10, 318 (1987). Because non-azeotropic mixtures may fractionate during the refrigeration cycle, they require certain hardware changes. The added difficulty in charging and servicing refrigeration equipment is the primary reason that non-azeotropic mixtures have been avoided. The situation is further complicated if an inadvertent leak in the system occurs during such use or service. The composition of the mixture could change, affecting system pressures and system performance. Thus, if one component of the non-azeotropic mixture is flammable, fractionation could shift the composition into the flammable region with potential adverse consequences.
The art is continually seeking new fluorocarbon based azeotrope-like mixtures which offer alternatives for refrigeration and heat pump applications. Currently, of particular interest, are fluorocarbon based refrigerants which are considered to be environmetally acceptable substitutes for the fully halogenated chlorofluorocarbons. The latter are implicated in causing environmental problems associated with the depletion of the earth's protective ozone layer. Mathematical models have substantiated that partially halogenated species, such as pentafluoroethane (HFC-125) and difluoromethane (HFC-32), will not adversely affect atmospheric chemistry; being negligible contributors to stratospheric ozone depletion and global warming in comparison to the fully halogenated species.
The substitute materials must also possess those properties unique to the CFC's including chemical stability, low toxicity, non-flammability, and efficiency in-use. The latter characteristic is important, for example, in refrigeration applications like air conditioning where a loss in refrigerant thermodynamic performance or energy efficiency may produce secondary environmental effects due to increased fossil fuel usage arising from an increased demand for electrical energy. Furthermore, the ideal CFC refrigerant substitute would not require major engineering changes to conventional vapor compression technology currently used with CFC refrigerants.
It is accordingly an object of this invention to provide novel azeotrope-like compositions based on pentafluoroethane and difluoromethane which are useful in cooling and heating applications.
Another object of the invention is to provide novel environmentally acceptable refrigerants for use in the aforementioned applications.
Other objects and advantages of the invention will become apparent from the following description.